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Red giant discovery For the first time, astonomers have detected the star system that caused a massive stellar explosion called a supernova.

The historic discovery provides surprising new insights into how these spectacular stellar deaths occur, and allows astronomers to refine calculations which use these types of supernovae to determine cosmic distances.

"By comparison, our observations last year of another Type 1a supernova progenitor system called SN 2011fe, showed no evidence of a red giant," he says.

"How could these events look so similar, if they had different origins?"

Total surprise

The scientists discovered PTF-11kx underwent at least two smaller eruptions called novae before finally ending its life in a supernova - the first evidence that Type 1a supernovae can occur in these systems.

"Taken together, these observations show that while all Type 1a supernovae seem similar, they can come from different kinds of stars," says Howell.

A nova is a massive explosion of material that's accreted onto the surface of a white dwarf from a companion star. While it is not unusual for star to undergo nova eruptions more than once, astronomers previously thought white dwarfs would lose more material in recurrent novae eruptions than they gain from their companion red giants and so couldn't accrete enough mass to go supernovae.

"We found incredibly strong calcium signals in the gas and dust surrounding the supernova, which is extremely unusual," says Howell.

"These clouds were moving too slowly to be coming from the supernova, and too quickly to be just stellar wind."

But for the novae theory to be true, faster moving material from the supernova should eventually catch up and collide with the gas and dust from the nova eruptions.

"That's exactly what happened," says Howell.

"At first, the calcium signal faded away, but 58 days after the supernova, there was a strong calcium signal burst, indicating supernova material had caught up, colliding with the nova material."

Cosmic candles

Although Type 1a supernovae are rare, occurring maybe once or twice a century in a typical galaxy, finding a Type 1a progenitor system like PTF 11kx is even more rare.

"You maybe find one of these systems in a sample of 1,000 Type 1a supernovae," says Peter Nugent from Lawrence Berkeley National Laboratory.

"This discovery gives us an opportunity to refine and improve the accuracy of our cosmic measurements."

When they explode Type 1a supernovae briefly become visible across the universe. Because they are so bright and explode with the same mass, they are used by scientists as standard candle markers to measure the size of the universe.